The present invention relates to components useful in making electrical connections to microelectronic elements such as semiconductor chips, and to methods of manufacturing such components.
Certain techniques for making semiconductor chip assemblies and similar microelectronic assemblies employ releasably attached leads. One such process is disclosed in commonly assigned U.S. Pat. No. 5,518,964, the disclosure of which is hereby incorporated by reference herein. In certain preferred embodiments described in the ""964 patent, a first element such as a dielectric layer in a connection component is provided with a plurality of elongated, flexible leads extending along a surface of the element. Each lead has a terminal end permanently attached to the first element and has a tip end offset from the terminal end. The tip ends of the leads may be releasably secured to the first element. A second element such as a semiconductor chip having contacts thereon is engaged with the first element or connection component, and the tip ends of the leads are bonded to contacts on the chip or second element. The elements are then moved away from one another so as to deform the leads and provide vertically extensive leads extending between the first and second elements, i.e., between the chip and the connection component. A compliant material may be introduced between the chip and the connection component.
The resulting structure allows the chip to move relative to the connection component without substantial stresses on the leads, and thus provides compensation for thermal expansion. The preferred structures can be readily tested and can be mounted on a further substrate such as a printed circuit panel or the like. Preferred embodiments of the processes disclosed in the ""964 patent can be used with chips or other microelectronic elements having large numbers of terminals. In the preferred processes, many leads can be deformed simultaneously. In particularly preferred processes according to the ""964 patent, the leads on a given connection component or first element may be connected to contacts on a plurality of chips such as an array of several chips or numerous chips formed as part of a wafer, so that many leads are deformed simultaneously.
In certain embodiments disclosed in the ""964 patent, the tip end of each lead is bonded to the surface of the first element by a small spot of a base metal such as copper interposed between the tip end and the surface. Typically, such a spot is formed by a process in which the leads are formed from an etch-resistant metal such as gold overlying a continuous layer of the base metal. The leads have wide portions at the tip and terminal ends. The component is then subjected to an etching process, as by exposing the component to a liquid etch solution which attacks the base metal so as to undercut the lead and remove the base metal from beneath the etch-resistant metal at all locations except at the terminal end and at the tip end. At the tip end, most, but not all of the base metal is removed from beneath the etch-resistant metal, leaving a very small spot of the base metal. The strength of the bond between the tip and the connection component surface is effectively controlled by the size of the spot. Thus, although the base metal may provide a relatively high bond strength per unit area or per unit length, it may still provide a weak attachment of the tip end of the lead end to the first element surface. Although structures such as frangible lead sections and small buttons can provide useful releasable attachments for the tip ends of the leads, some care is required in fabrication to form these features. Formation of spots of base metal of uniform size beneath the terminal ends of leads on a large connection component requires control of the etching process. Moreover, any variation in the strength of the bond between the base metal and the surface will result in a corresponding variation in the strength with which the tip ends of the leads are held to the surface.
As described in PCT International Publication WO 94/03036, the disclosure of which is hereby also incorporated herein by reference, a connection component may incorporate a support structure such as a polyimide or other dielectric layer with one or more gaps extending through such layer. Preferably, the support structure incorporates one or more flexible or compliant layers. The connection component may further include leads extending across the gap. Each lead has a first or terminal end permanently secured to the support structure on one side of the gap, and a second end releasably attached to the support structure on the opposite side of the gap. In preferred processes as taught by the ""036 publication, the connection component is positioned on a semiconductor chip or other microelectronic element. Each lead is engaged by a bonding tool and forced downwardly into the gap, thereby detaching the releasably connected second end from the support structure. The leads are flexed downwardly into the gap and bonded to the contacts on the chip or the microelectronic element. Preferred connection components and processes according to the ""036 publication also provide highly efficient bonding processes and very compact assemblies. The finished products provide numerous advantages such as compensation for thermal expansion, ease of testing and a compact configuration.
Other structures disclosed in the ""036 publication and in the ""964 patent employ frangible lead sections connecting the releasable end of each lead to another structure permanently mounted to the support structure or first element. Frangible sections can also provide useful results. However, such frangible elements are most commonly formed by using the photo-etching or selective deposition processes used to form the lead itself to form a narrow section. The minimum width at the narrow section, can be no less than the smallest width formable in the process. As the other portions of the lead adjacent the narrow section must be wider than the narrow section, these other portions must be larger than the minimum attainable in the process. Stated another way, the leads made by such a process generally are wider than the minimum line width attainable in the formation process. This limits the number of leads which can be accommodated in a given area.
In other embodiments disclosed in the ""036 publication, the first or permanently mounted terminal end of a lead may have a relatively large area, whereas the second or releasably mounted end of the lead overlying the support structure may have a relatively small area, so that such second end will break away from the support structure before the first end when the lead is forced downwardly by the bonding tool. This arrangement requires control of the dimensions of the ends to control the area of the bond between the lead end and the support structure and also requires a lead wider than the smallest element formable in the process.
As described in the ""036 publication, and as further described in commonly assigned International Publication WO 97/11588, the disclosure of which is also incorporated by reference herein, leads used in these and other microelectronic connection components may include polymeric layers in addition to metallic layers. The polymeric layers structurally reinforce the leads. For example, certain leads described in the ""588 publication incorporate a pair of thin conductive layers such as metallic layers overlying opposite surfaces of a polymeric layer. One conductive layer may be used as a signal conductor, whereas the opposite conductive layer may act as a potential reference conductor. The composite lead thus provides a stripline extending along the lead. A stripline lead of this nature can provide a low, well-controlled impedance along the lead, which enhances the speed of operation of the circuit formed by the connection component and the associated microelectronic elements. The potential reference conductor also helps to reduce crosstalk or undesirable inductive signal coupling between adjacent leads. Despite all of these improvements, still further improvements would be desirable.
One aspect of the present invention provides methods of making connection components. A method according to this aspect of the invention desirably includes the steps of providing a starting structure including one or more metallic leads overlying a polymeric dielectric layer, and etching portions of the dielectric layer disposed beneath said one or more leads by contacting the starting structure with an etchant, most preferably a gaseous etchant such as a plasma of a reactant gas including oxygen or other oxidizing gas. Typically, in the starting structure the leads overlie a first surface of the dielectric layer. The etching step may be performed by exposing the first surface, with the leads thereon, to the etchant. The etching step most preferably is performed so as to leave only certain parts of the said leads connected to the dielectric layer by etch-defined polymeric connection regions smaller than such parts. Thus, after the etching step, the leads are spaced vertically from the etched surface of the dielectric layer. The connection regions form polymeric connecting elements integral with the dielectric layer and extending vertically between the dielectric layer and the overlying regions of the leads, referred to herein as the attachment regions of the leads.
The polymeric connection regions or connecting elements provide reliable, readily releasable connections between the attachment regions of the leads and the dielectric layer. Regardless of the degree of adhesion between the polymeric layer and the material of the leads, the force required to release a connection cannot exceed the tensile strength of the polymeric connecting element. This is controlled by the cross-sectional area of the polymeric connecting element. This area can be controlled accurately in the etching process.
Each lead may include first and second ends, and an elongated region extending between these ends. The etching step may be performed so as to detach the elongated region from the dielectric layer, and so as to leave one end of the lead releasably connected to the dielectric layer by such a connecting element. The other end of the lead may be left permanently anchored to the dielectric layer. For example, the first ends of the lead and the underlying portion of the polymeric layer may be covered with a mask during the etching process, so that the polymeric layer remains substantially unetched and the first ends remain securely anchored to the polymer layer. The second ends of the leads and the underlying portions of the polymeric layer may be exposed to the etchant so as to remove parts of the polymeric layer and form the polymeric connecting elements beneath the second ends of the leads.
A further aspect of the invention provides a microelectronic connection component comprising a support structure including a polymeric dielectric layer having a surface extending in horizontal directions; one or more metallic conductive structures such as leads overlying said surface of said dielectric layer, the conductive structures having attachment portions vertically spaced from said surface; and polymeric connecting elements integral with said dielectric layer extending between the surface and the attachment portions of said conductive structures. The attachment portions of the leads overly the polymeric connecting elements. Each such connecting element has at least one horizontal dimension smaller than the corresponding horizontal dimension of the attachment portion overlying that connecting element. The connecting elements preferably form releasable connections between the attachment portions of the leads and the support structure. Components according to this aspect of the invention can be fabricated according to the processes discussed above.
A further aspect of the invention provides methods of making microelectronic connection components comprising the steps of: providing a support structure and one or more leads mounted to said support structure; and depositing a dielectric material on the leads. Most preferably, the leads are deformable or movable with respect to the support structure. The depositing step preferably is performed so that the deposited dielectric material provides a continuous jacket extending entirely around the lead over at least a portion of its length. For example, portions of the leads may project from an edge of the support structure or project across gaps in the support structure, and a continuous jacket may be provided in these portions of the lead. The process can provide microscopic leads on connection components with insulating jackets typically provided only on much larger leads such as conventional wires. The depositing step may be performed by means of an electrophoretic deposition bath.
According to a further aspect of the invention, the process may include the additional step of depositing a conductive layer such as a metallic layer over the deposited dielectric. The metallic layer thus forms a reference conductor extending coaxially with the lead but insulated therefrom by the dielectric jacket. In effect, each lead is converted to a miniature coaxial cable.
Yet another aspect of the invention provides microelectronic connection component including a support structure and one or more leads attached to the support structure. Each lead has an elongated section movable with respect to the support structure. A jacket of a dielectric material surrounds each said lead over at least a part of the elongated section of that lead. The component preferably further includes reference conductors surrounding and extending coaxially with the elongated sections of said leads and insulated therefrom by the jackets of dielectric material, the reference conductors including a coating of an electrically conductive material overlying the dielectric jackets. The elongated sections of the leads preferably have cross-sectional dimensions less than about 100 xcexc, and typically about 50xcexc or less, whereas the dielectric material may be about 12-50 xcexcm thick. The reference conductors may be electrically connected to potential plane elements on the support structures, such as ground or power planes.
The insulated leads provide immunity to accidental short-circuiting during or after connection with the microelectronic component. In the embodiment incorporating the conductive jackets, the leads act as miniature coaxial cables, and provide well-controlled impedance which enhances signal propagation and permits operation at high frequencies. The miniature coaxial cables also provide outstanding immunity to electromagnetic interference such as cross-talk between adjacent leads. In a further variant, each lead and the surrounding conductive jacket may serve as the two conductors of a differential signaling circuit.